The invention relates to a tensioning roller for centred accommodation of a medium to be treated according to a pattern with high-energy radiation, at least comprising a central shaft around which is placed a supporting element for the medium to be treated, which is expandable in the peripheral direction and in the expanded state is arranged symmetrically round the shaft and, viewed in the radial direction going out from the shaft comprises a supporting cylinder, an inflatable bag made of elastic material, a thin-walled metal cylindrical sleeve disposed around it, centring means for centring said supporting cylinder, inflatable bag and cylindrical sleeve, and means for the supply and discharge of inflation medium for the inflatable bag.
Such a tensioning roller is generally known and is used, for example for designing rotary screen printing stencils.
Such a rotary screen printing stencil is formed by a thin-walled, seamless metal screen material, for example a nickel screen material, said screen material being coated with a light-sensitive coating composition. When pumped up, the rubber bag disposed inside the cylindrical screen gives the rotary screen printing stencil a suitable cylindrical shape, the fastening of the bag being such that the central shaft of the cylinder sleeve coincides with the central shaft of the supporting cylinder around which the inflatable bag is arranged. If a diapositive film is tensioned with the emulsion side lying against the light-sensitive coating present on the screen and is then exposed to ultraviolet radiation, the light-sensitive coating, for example a polyvinyl alcohol/ammonium bichromate coating, is hardened, for example at the light-transmitting parts of the diapositive film, in such a way that it can no longer dissolve in water.
At the places where the coating has not hardened, i.e. where the coating has not been struck by actinic rays, for example ultraviolet radiation, the coating has not hardened, it can be removed using water. A negative picture of the positive pattern present in the film is formed in this way.
During printing which such a printing block, for example on a rotary screen printing machine, at all points of the screen which correspond to the blackened parts in the positive film printing compound is allowed through the screen, so that a positive print of the original positive pattern is formed on the surface of the substrate to be printed.
The element which in the above description of the tensioning roller corresponds to the medium to be exposed is in this case the light-sensitive coating present on the screen.
Of course, the supporting cylinder mentioned in the above description can also be a closed cylinder, and the device in which the tensioning roller is accommodated can be used for treating, say, a film material with light according to pattern.
Such a tensioning roller can also be used, for example, for burning open with a laser beam the perforations of a screen printing stencil material closed with a sealing lacquer.
In view of the metal nature of the cylindrical sleeve, such known tensioning rollers have an essentially fixed peripheral measurement which can be varied within narrow limits only by great variation in the inflation pressure applied.
For many applications a greater variation possibility would be desired, but a temporarily applied peripheral enlargement must not lead to permanent deformation of the material of the cylindrical sleeve.
The object of the present invention is to produce a tensioning roller of the type mentioned, but in which through variation of the pressure applied for the inflation medium the periphery of the cylindrical sleeve can be varied within considerably wider limits than has been the case hitherto.